1. Field of the Invention
The present invention relates to an automatic needle thread supply control system for a sewing machine capable of automatically controlling the amount of supply of a needle thread to a thread take-up member from a thread supplying source in response to various conditions of sewing.
2. Description of the Prior Art
An apparatus described in the U.S. Pat. No. 4,215,641 is proposed as the conventional automatic needle thread supply control system for a sewing machine. In the system, a needle thread clamping means is constituted so as to comprise a solenoid, and the needle thread is clamped by exciting the solenoid and the needle thread is released by demagnetizing the solenoid, and thereby supply and stop of the needle thread is controlled.
However, when a sewing machine is operated at a high speed, the operating time of the needle thread clamping means allowed for clamping and releasing the needle thread becomes short, and excitation and demagnetization are required to be performed in a short time. However, because of having a peculiar delay of operation, the solenoid sometimes cannot respond accurately to a signal commanding switch-over to excitation or demagnetization. Consequently, it cannot control accurately the amount of supply of the needle thread, and a problem remains that no beautiful seam line is formed because of variation in the tightened state of each seam.
Furthermore, since the solenoid has to be kept excited over a period during which the needle thread is clamped, such problems are raised that the solenoid is heated and consumes a relatively high power.